Recently, unmanned aerial vehicles/systems (UAVs/UASs) have found increasing civilian and commercial applications, such as law enforcement, search and rescue, and precision agriculture. These commercial applications require UAVs to operate in civilian airspace. Central to operating UAVs in civilian airspace is the challenge of meeting the stringent safety standards set by the Federal Aviation Administration (FAA). In 2012, the United States Congress passed H.R.658—the FAA Modernization and Reform Act—in order to facilitate the safe integration of UASs into the national airspace. In particular, section 332 of H.R.658 mandates the FAA to “provide for the safe integration of civil unmanned aircraft systems into the national airspace system as soon as practicable, but not later than Sep. 30, 2015.” In August 2016, the FAA's Final Rule for 14 CFR Part 107 became effective and laid out rules for UAS operators to follow. This brings in legislative and policy dimensions to what is academically seen as a technical challenge.
To put this challenge in perspective, consider the current safety standards set by the FAA for manned commercial aircraft. In order for a manned commercial aircraft to be certified, there should be no more than one catastrophic failure per 109 flight hours. Commercial aircraft manufacturers, such as Boeing, meet the 10−9 failures-per-flight-hour standard by utilizing hardware redundancy. For example, the Boeing 777 has 14 spoilers each with its own actuator: two actuators each for the outboard ailerons, left and right elevators, and flaperons; and three actuators for the single rudder. On the other hand, most civil UAVs have reliabilities that are orders of magnitude below the standard of 10−9 failures-per-flight-hour. For instance, the UAV Research Group at the University of Minnesota (UMN) operates an Ultra Stick 120 aircraft (described further below) with single-string, off-the-shelf components. A comprehensive fault tree analysis yielded a failure rate of 2.2×10−2 failures-per-flight-hour for this aircraft.
UAVs have such low reliability for two main reasons. First, most on-hoard components are not very reliable because they are low-cost. Second, most on-board components have little to no hardware redundancy. Consequently, there are single points of failure that can lead to catastrophic failure. Safe integration of UAVs into the national airspace requires increases in UAV reliability. However, the solutions adopted to increase the reliability of manned commercial aircraft are not downwardly scalable to UAVs, in particular, hardware redundancy must be used judiciously because of the costs associated with size, weight, and power. Analytically redundant solutions, such as robust and fault-tolerant control, have the potential to bridge the gap between commercial aircraft, that almost entirely use hardware redundancy, and current UAVs, that are almost entirely single-string designs.